In a magnetic resonance imaging apparatus (hereinafter referred to as an “MRI apparatus”), an object to be examined as a target to be examined is placed within a uniform static magnetic field, and a nuclear magnetic resonance signal (hereinafter referred to as an “NMR signal”) from the object to be examined is detected by utilizing a nuclear magnetic resonance phenomenon that is caused by nuclei of atoms making up a body of the object to be examined when an electromagnetic wave is irradiated to the object to be examined. By reconstructing an image from the detected NMR signal, a magnetic resonance image (hereinafter referred to as an “MRI image”) representing physical properties of the object to be examined is obtained. To provide position information in such an imaging process, a gradient magnetic field is applied in superimposed relation to the static magnetic field.
In a vertical magnetic field system in which the direction of the static magnetic field is orthogonal to the direction of a body axis of the object to be examined, a pair of gradient magnetic field coils are arranged in vertically opposed relation inside a pair of static magnetic field generating sources (i.e., within a uniform static magnetic field) which are also arranged in vertically opposed relation. Further, because the gradient magnetic field is generated in each of three-axis directions orthogonal to each other, the pair of gradient magnetic field coils are each constituted by three sets of pair of magnetic field generating coils.
A gradient magnetic field power supply is connected to each gradient magnetic field coil such that a pulse-like current is applied to the gradient magnetic field coil at the proper timing and voltage depending on conditions which are required when imaging and examination are carried out in the MRI apparatus. However, when the pulse-like current is applied to the gradient magnetic field coil, the gradient magnetic field coil is vibrated by the action of the Lorentz's force, thus producing noise.
As the prior art for solving that problem, there is known an MRI apparatus having a structure in which the gradient magnetic field coil is put in a recess formed in a pole piece for sound insulation (see Patent Document 1).
In the prior art disclosed in Patent Document 1, the gradient magnetic field coil is attached to the pole piece while a D-piece made of a soft material, e.g., rubber, is interposed between them, whereby the vibration of the gradient magnetic field coil is prevented from being transferred to the pole piece.
In the above-cited Patent Document 1, however, if the D-piece is too soft, a problem arises in that a position variation caused by the vibration of the gradient magnetic field coil is increased and fluctuation of the gradient magnetic field caused by the increased position variation brings about an image artifact.
To solve such a problem, Patent Document 2 discloses a technique of distributing a load imposed on the gradient magnetic field coil to a large number of elastic bodies so that a displacement of each elastic body is reduced to a sufficiently small amount, thereby suppressing fluctuation in distribution of the gradient magnetic field.
Patent Document 3 also discloses a technique for preventing noise caused by the vibration of the gradient magnetic field coil.
In Patent Document 3, the noise is reduced by setting a spectrum characteristic of the waveform of a current applied to the gradient magnetic field coil so that the intensity of a component having a frequency f matched with the natural frequency of the gradient magnetic field coil including a holding member is substantially 0.
Further, Patent Document 3 discloses a method for modifying parameters related to the natural frequency, such as an axial length and a fixed position of the holding member, so that in a certain particular spectrum component, a frequency at which the intensity is substantially 0 is matched with the natural frequency of the gradient magnetic field coil.
Meanwhile, Patent Document 4 discloses an MRI apparatus in which a ferromagnetic shim is arranged between a static magnetic field generating source and the gradient magnetic field coil to increase uniformity of the static magnetic field, thereby improving image quality.
Patent Document 1: JP,A 11-137535
Patent Document 2: Japanese Patent 3156088
Patent Document 3: JP,A 10-201735
Patent Document 4: JP,A 2002-360537
However, the technique disclosed in Patent Document 2 has the problems as follows. In order to reduce the displacement of each elastic body to the sufficiently small amount, a large number of elastic bodies have to be arranged over a wide area. Therefore, although the noise caused by the vibration of the gradient magnetic field coil can be suppressed, a difficulty occurs in improving image quality because an area for arrangement of image quality improving means, such as disclosed in Patent Document 4, is limited.
Also, the technique disclosed in Patent Document 3 has the problems as follows.
In the MRI apparatus, imaging is required to be performed by applying the gradient magnetic field that has various frequency components ranging from a normal sequence to a high-speed sequence. Therefore, the imaging performed using only the particular current waveform, which has been adjusted so as to suppress the vibration, is not adaptable for various sequences.
Further, the technique of modifying the parameters of the holding member also has a difficulty in preventing noise and improving image quality because it is practically impossible to modify the parameters, such as the axial length and the fixed position of the holding member, for each of the various sequences.
In an aspect of this disclosure, there is provided a magnetic resonance imaging apparatus which can suppress noise caused by vibration of a gradient magnetic field coil and can improve image quality.
In another aspect of this disclosure, there is provided a magnetic resonance imaging apparatus comprising static magnetic field generating means for generating a static magnetic field in an imaging space, gradient magnetic field generating means for generating a gradient magnetic field in the imaging space, RF magnetic field generating means for generating an RF magnetic field, signal receiving means for detecting a nuclear magnetic resonance signal, and signal processing means for reconstructing an image by using the detected nuclear magnetic resonance signal.
In addition to the above-mentioned components, the magnetic resonance imaging apparatus further comprises static magnetic-field non-uniformity correcting means disposed between the static magnetic field generating means and the gradient magnetic field generating means, including a plurality of static magnetic-field non-uniformity correcting members, and having a plurality of holes formed therein; and a plurality of vibration isolating members disposed in the plurality of holes formed in the static magnetic-field non-uniformity correcting means and suppressing transfer of vibration generated in the gradient magnetic field generating means to the static magnetic field generating means.
In another aspect of this disclosure, there is provided a magnetic resonance imaging apparatus comprising static magnetic field generating means for generating a static magnetic field in an imaging space, gradient magnetic field generating means for generating a gradient magnetic field in the imaging space, RF magnetic field generating means for generating an RF magnetic field, signal receiving means for detecting a nuclear magnetic resonance signal, and control means for reconstructing an image by using the detected nuclear magnetic resonance signal and for generating the gradient magnetic field and the RF magnetic field in accordance with a plurality of pulse sequences.
In addition to the above-mentioned components, the magnetic resonance imaging apparatus further comprises vibration suppressing means for modifying a frequency characteristic or a transfer characteristic of vibration caused when the gradient magnetic field generating means is vibrated.
The configuration of the above-mentioned magnetic resonance imagine apparatus makes it possible to secure areas for arrangement of two means, i.e., the static magnetic-field non-uniformity correcting means and the vibration suppressing means, to reduce noise caused by the vibration of the gradient magnetic field generating means, and to suppress deterioration of image quality.
In addition, such magnetic resonance imaging apparatus makes it possible to suppress generation of the noise caused by the vibration of the gradient magnetic field coil in a manner adapted for each of various sequences, and to improve image quality.